Heat transfer coils of the type involved herein are known, for example, from FIG. 9 of U.S. Pat. No. 3,318,376. I have found that the generally U-shaped conduit with flared side edges as shown in FIG. 9 of said patent is objectionable because of the bending stresses introduced into the conduit as a result of the flared side edge portions. The bending stresses introduced into the conduit is a result of cyclical pressure and temperature changes which results in early failure due to fatigue. While the prior art uses conduits which are semi-circular in cross-section so as to have arcuate periphery of 180.degree. or more, I have found that a conduit which has an arcuate periphery of less than about 150.degree. produces a number of advantages not attainable with the prior art conduits.